Graphene phototransistors get gain

Graphene makes a nearly ideal photodetector: It absorbs light over a broad range of wavelengths—from the UV to the far-IR—and ballistically conducts, even at room temperature, any electrons photoexcited into the conduction band. But as a monolayer sheet of carbon atoms, graphene can absorb only 2.3% of the incident light. To boost that absorption, Gerasimos Konstantatos , Frank Koppens , and their team at the Institute of Photonic Sciences in Barcelona, Spain, coated the graphene with a 60-nm-thick film of light-absorbing lead sulfide quantum dots. As proof of concept, they fabricated an ultrahigh-gain phototransistor, shown schematically. Photons incident on the PbS generate electron–hole pairs that separate under a built-in electric field at the interface. Holes are swept into the graphene, where they conduct to the drain in a few nanoseconds. Meanwhile, the electrons drift upward and become trapped for a few hundred milliseconds in the dots’ surface states, where they act like a gate to modulate the current from source to drain. By charge conservation, additional holes rush in from the source to replenish those lost from graphene to the drain during the time an electron remains trapped. The result is a device with an enormous gain of 10⁸ charge carriers per photon. That gain comes at the price of slow switching frequencies, typically under 100 Hz. Even so, it could be useful for applications such as visible- and IR-sensitive cameras. (G. Konstantatos et al., Nat. Nanotechnol., in press, doi:10.1038/nnano.2012.60 .)—R. Mark Wilson